Ion selective electrodes (ISE), in general, are electrochemical sensors, in the case of which the relative change of the equilibrium Galvani voltage between a measured medium and a sensing electrode IS preferably effected by the activity change predominantly of a certain kind of ion. Such ion selective electrodes allow a relatively simple and fast determination of ion concentrations in different media, even e.g. in turbid and colored solutions. Ion selective electrodes are applied, for example, in process liquid analysis and in wastewater analysis.
Potentiometric measurements with ion selective electrodes metrologically correspond largely to the classic pH measurement technique based on pH glass membrane electrodes. Referencing a reference potential of a reference electrode having an essentially constant potential, e.g. the well known Ag/AgCl-electrode, the concentration of a measured ion in a liquid can be determined with high accuracy by means of a high impedance voltmeter with little in the way of apparatus.
Currently, besides glass membranes, membranes known as solid body or polymer membranes are also used as ion selective components of such electrodes. Polymer membranes frequently include a plasticizer as a lipophilic solvent, a salt of the kind of ion to be measured, wherein the salt has a lipophilic counter ion, and a polymer material as a network former for the holding the membrane together. Frequently in the case of cation selective membranes, an ionophore is also present in the membrane. Ion selective electrodes of this type are described, for example, in “Ion selective electrodes,” J. Koryta and K. Stulik, Cambridge University Press, 1983, S. 61, or in “Das Arbeiten mit Ionenselektiven Elektroden,” K. Cammann, H. Galster, Springer, 1996.
Ion selective electrodes, especially ion selective electrodes based on solid body or polymer membranes, in general, do not respond specifically only to the kind of ion to be measured, also referred to as measured ions in the following, but, instead also to other ions, which are referred to as disturbing ions. Thus it is known, for example, that ammonium (NH4+) selective electrodes, also referred to in the following as an ammonium-ISE, also respond to the chemically similar potassium ion (K+). In a similar manner a nitrate ion (NO3−) selective electrode, in the following also referred to as a nitrate-ISE, also responds to chloride ions (Cl−). This is especially disturbing for ISE applications in water analysis, since chloride is present almost universally in drinking water and in wastewater and, above a certain minimum concentration, degrades the measuring of nitrate concentrations.
For example, the selectivity of an ammonium-ISE is, with reference to the molar concentration, 1:10 relative to potassium. The ammonium-ISE thus gives in the presence of a determined ammonium concentration the same measurement signal as in the presence of a ten times as high concentration of potassium. A nitrate-ISE possesses a selectivity of 1:100 relative to chloride with reference to the molar concentration. Thus a 100 times as high concentration chloride is required in order for the nitrate ISE to output the same measurement signal as in the case of a determined nitrate concentration.
In wastewater analysis, the parameters nitrate nitrogen (Nitrate-N), i.e. the mass of nitrogen bound in nitrate ions present in a water sample with reference to the volume of the water sample, and ammonium nitrogen (Ammonium-N), i.e. the mass of nitrogen bound in ammonium ions present in a water sample with reference to the volume of the water sample, play an important role since they are limited by regulations and/or require the paying of fees. Referencing the disturbing influence of a determined mass concentration of chloride ions to the parameter nitrate-N, the selectivity of a nitrate-ISE set forth above is 1:440, for instance. Referencing the disturbing influence of a determined mass concentration of potassium ions to the parameter ammonium-N, the selectivity of an ammonium-ISE is 1:28, for instance.
The parameter ammonium-N in wastewaters of wastewater plants is frequently in a range between 0.1 and 20 mg/l ammonium-N. In municipal wastewater, the potassium concentration typically is in the range of 20 mg/l. In a selectivity of the ammonium-ISE of 1:28 compared to a potassium ion concentration present in the sample, for instance, a measured value of the ammonium-N is obtained, which is around 1 mg/l too high. This is not always acceptable.
There are therefore efforts being made to correct measured values coming from ion selective electrodes and corrupted by the presence of disturbing ions.
An opportunity for compensating cross sensitivities of ion selective electrodes involves measuring the concentration of the disturbing ions, chloride or potassium ions for example, by means of additional electrodes and taking into consideration the selectivity of the particular measuring electrode, thus the ammonium-ISE or the nitrate-ISE for example, in order to correct for the disturbing influence of the measured disturbing ion concentration on the measurement signal of the measuring electrode. An additional selective ISE for the expected disturbing ions is an option for the additional electrode, e.g. an ion selective electrode for chloride ions (chloride-ISE) or an ion selective electrode for potassium ions (potassium-ISE). Such an additional ISE is also referred to as a compensation-ISE of the measuring arrangement.
This solution, however, has disadvantages: The additional ISE must be procured, maintained and, in determined intervals, for example, be renewed by replacing the ion selective membrane. Especially, the additional ISE, like the ISE provided for monitoring the measured ions of real interest, must be calibrated or adjusted at regular intervals. This leads to an additional maintenance effort and also to increased costs in the manufacture of the measuring arrangement.
In such a case, it is to be noted that an incorrectly adjusted compensation-ISE can lead to defective measured values. A measuring device with a measuring ISE and a compensation-ISE consequently has more to it, is more complicated to construct and, finally, more susceptible to disturbances.
Another very simple and frequently sufficient correction method uses the measuring device ISEmax CAS40 available from the present assignee for measuring and monitoring ammonium concentration using an ammonium ISE in municipal wastewater treatment plants. In such a case, the fact is utilized that the concentration of the potassium ions, which act as disturbing ions for the ammonium-ISE, fluctuates in the water of municipal wastewater treatment plants in general only a little. Therewith the disturbing influence of the potassium ions is essentially constant and can therefore be compensated by lessening the measured value obtained from the ammonium-ISE by a fixed, predetermined amount. In the case of an average concentration of potassium ions of 20 mg/l, for example, an inflated reading of about 1 mg/l ammonium-N is caused. This value can, for example, be fixedly predetermined as a correction value and subtracted from each concentration value derived from the measurement signal delivered from the ammonium ISE to form a corrected measured value. To the extent that fluctuations of the potassium ion concentration occur in a relatively narrow range of about 15 mg/l and 25 mg/l, for example, in the liquid to be monitored, there results only a relatively small error, which can be accepted in many applications. Ideally, the correction value is set by a skilled technician who can rely on experiences with the disturbing ion concentrations present at the actual measuring point and their influence on the accuracy of measurement of the measuring device.